Water absorbs more heat than any other substance, so that this quality is utilized; but the water, if not kept in motion, when applied to such a highly-heated surface as an engine cylinder, would be converted into superheated steam, and would then be of no further value.
Water Absorption.—This necessitates a constant and intermitting motion, so that the more rapidly the water moves, the less it will become heated. At the same time, means must be provided to cool the water in its circuit back to the engine, and the most efficient means to accomplish this is to provide a radiator at the forward end of the machine.
The circulating system, together with the radiator, will be described under their proper headings.
Engine Types.—There are two distinct types of engine, one called the two-cycle, and the other the four-cycle. Cycle has reference to a period or turn, in which certain mechanical operations are completed in regular order so to form a succession of events.
The Four-Cycle Engine.—These events in a four-cycle engine require the crank to make two complete turns, the order being as follows: Starting with the explosion of the charge, the first element in the cycle, is the downward movement of the piston (expansion); second, the return of the piston to the upper end of the cylinder (exhaust); third, the downward movement of the piston, on its second revolution, and the drawing in of a fresh charge of fuel (suction); and fourth, the return stroke which compresses the fuel for driving the piston down the next stroke (compression).
The Two-Cycle.—The two-cycle engine, at the explosion, sends the piston downwardly, and as the crank case and cylinder are connected up together so as to form an air tight receptacle, within which the crank and shaft turn, the downward movement of the piston compresses all the gas which has been previously drawn into the crank case.
When the piston reaches the extreme limit of its downward movement, it uncovers a port in the side wall of the cylinder, so as to afford an outlet for the gases of combustion, and immediately thereafter the piston also uncovers a duct that leads from the crank case, so that the previously compressed gases, as stated, rush in, and this inward movement of the fresh gas, also facilitates the movement of the burnt gases at the opposite side.
Compression.—When the piston starts on its return stroke, or upward movement, it compresses the charge thus received, and when the piston nears the upper end of its stroke the sparking mechanism again explodes it, so that the cycle is formed by the two operations, performed by a single turn of the crank shaft.
This latter type of engine is not used to a great extent. It has the advantage that no valves are used, except the one at the inlet of the gas to the crank case, and no stems, push rods, cam shafts, or springs are required to control the movements of the fresh and burnt gases. Aside from that such engines weigh considerably less than the four cycle type.
Economy of Four-Cycle Engine.—On the other hand, the four cycle is more economical, because there is more time for the admission of the fuel, and for exhausting the gases. Furthermore, it is obvious that in a two cycle engine more or less of the fresh fuel gas is mixed with and is discharged from the cylinder with the burnt gases.